1. Field of the Invention
The present invention relates to a method of crystallization through vacuum concentration. More specifically, it relates to a method of crystallization through vacuum concentration in which crystals having a uniform particle size within a narrow particle size distribution range can be obtained by periodically varying up and down or swing the temperature of the slurry (mass temperature) during concentration of the slurry (i.e., the crystallization mother liquor) from which crystals are to be precipitated.
2. Background Art
A method of crystallization by controlling the particle size distribution (i.e., with the particle size distribution being controlled) in which crystals having a uniform particle size in a narrow particle size distribution range can be obtained is useful in, for example, the following fields, and the development of an excellent method of crystallization by controlling the particle size distribution has been in high demand. The said following fields are as follows;
(1) When a crystal product has to comply with a particle size standard, the crystallization is controlled in such that the particle size of crystals precipitated is arranged within the standard, whereby the yield (i.e., the standard pass rate) of the product is improved.
(2) The crystallization is controlled in such that formation of fine crystals is suppressed, whereby the solid-liquid separability of the crystals precipitated is improved and the purity thereof is, in turn, increased (by decreasing the mother liquor adhering thereto).
(3) Likewise, the separability is improved to reduce the time required for separation, which, in turn, improves the productivity.
(4) When the solid-liquid separation using a cloth filter is repeated, a dense bed layer remains on the filter cloth, so that the separability is gradually decreased. Accordingly, the separability has to be restored through washing or the like. At this time, however, the content moved into the washings becomes a loss. If the crystal separability gets better, this loss is reduced, and the yield is increased, contributing to the improvement of the productivity.
In this connection, as the method of crystallization by controlling the particle size distribution, or with the particle size distribution being controlled, for example, the following methods have been so far proposed.
(a) Method described in JP-B-49-29,821:
This relates to a method in which the alumina content is extracted from an aluminous mineral through treatment with sulfuric acid, and aluminum sulfate is produced as high-quality crystals from the extract. Specifically, it is a method of producing aluminum sulfate, comprising, in combination, a first step of precipitating aluminum sulfate crystals by cooling a part of a sulfuric acid-acidic aluminum sulfate solution of a high temperature which has been obtained by treating an aluminous mineral with sulfuric acid, a second step of increasing the temperature of the solution in which the aluminum sulfate crystals have been precipitated to such an extent that the crystals therein are partially dissolved, maintaining this temperature for a predetermined period of time, then recooling the solution to precipitate additional aluminum sulfate crystals, and repeating this procedure to obtain hexagonal plate crystals, and a third step of adding the resulting hexagonal plate crystals to the saturated or nearly saturated sulfuric acid-acidic solution which is the residual portion of the aluminum sulfate solution obtained in the first step to precipitate the aluminum sulfate hexagonal plate crystals and separate the same.
It is further stated that according to this method, there is no need to select an aluminous ore having a high alumina content, crystals of aluminum sulfate formed are thick hexagonal plate crystals having a good filterability, and impurities adhering to the crystals can be easily removed through washing.
(b) Method described in JP-A-62-247,802:
This relates to a batchwise cooling crystallization method (i.e., a batchwise method of crystallization by cooling), and more specifically to a method in which coarser crystals are obtained. Specifically, it is a batchwise cooling crystallization method characterized in that after cooling starts and a part of the solute is crystallized, cooling is stopped, and heating is conducted; and before the crystals precipitated are completely dissolved, the heating is stopped, and recooling is conducted.
The reasons for proposing such a method are described as follows. In the batchwise cooling crystallization, the conventional method employed to increase the crystal size includes adjustment of cooling rate, adjustment of solute concentration, adjustment of solute composition, and the like.
Further, crystallization medium is some times studied. However, in case that upon studying these methods the improvements are not expected or it is difficult to change the composition and the concentration in view of the process and there is no room for investigation, the production might be impossible, but this inventive method is a simple, quite useful crystallization method upon solving the problems.
(c) Method described in JP-A-5-111,602
This relates to a method of forming crystal particles having a uniform particle size, which method is required for production of sugar and the like, and more specifically to a method of crystallizing crystal particles from a solution, which comprises conducting crystallization in a crystallizer, while measuring the particle size distribution of the crystal particles with a particle size meter fitted in the crystallizer, and decreasing the degree of saturation of the solution based on the detection when detecting that the number of particles in the particle size distribution exceeds a predetermined value.
The reasons for proposing such a method are described as follows. That is, for example, in the conventional sugar crystallization method, it was difficult to obtain crystal particles having a uniform particle size at good efficiency. That is, in order to shorten a crystallization time, a growth rate of crystal particles can be increased by increasing the degree of supersaturation of a solution. However, when the degree of supersaturation of the solution is increased too much, mimetic crystals, i.e., crystal nuclei newly formed besides the seed crystals, are generated to make non-uniform the particle size of the crystal particles. In order to remove the mimetic crystals formed, a solvent is added to the solution to reduce the degree of supersaturation, making it possible to remove the mimetic crystals by dissolving. However, when such re-dissolution is conducted after the mimetic crystals are grown, the time for re-dissolution is required, and the desired crystal particles are also dissolved. Accordingly, the overall crystallization time is increased to make impossible the efficient production. Therefore, for obtaining crystal particles having a uniform particle size, it is important that while observing the condition of the crystallization, a solvent and a solution are added or removed to control the solution to an optimum degree of supersaturation. The crystallization has been so far conducted based on the visual observation and the experiences of workers. Or, the degree of supersaturation has been controlled for automation, by measuring the viscosity correlated with the density and the particle size of crystal particles using a densitometer. However, such controlling is problematic in that the reproducibility in the crystallization step is poor. Incidentally, the particle size of the crystal particles in the solution within a crystallizer cannot accurately be measured with the conventional particle size meter with which the precipitation rate of the particles is measured. This is because when the solution and the crystal particles are withdrawn from the crystallizer for the measurement, the temperature and the like are changed to change the degree of supersaturation, so that the dissolution and the growth of the crystal particles occur. Meanwhile, the inventive method has dissolved such problems, and it aims to provide a crystallization method in which crystal particles having a uniform particle size can be obtained with good reproducibility (refer to paragraphs [0003] to [0007] of the same document).
Nevertheless, such a method of crystallization by controlling the particle size distribution as proposed so far still involves various problems (to be described later), and the further improvement has been in demand.
In view of the background of such conventional techniques, it is one object of the present invention to provide an excellent method of crystallization by controlling the particle size distribution i.e., with the particle size distribution being controlled, which method is free from the defects of the conventional methods.
The present inventors have assiduously conducted investigations to achieve the and other objects, and have consequently found that while the crystallization mother liquor is concentrated to precipitate crystals, the temperature of the slurry, i,e., the mother liquor, (mass temperature) is periodically varied up and down during the concentration whereby crystals having a uniform particle size in a narrow particle size distribution can be obtained. These findings have led to the completion of the present invention.
That is, the present invention relates to a method of crystallization with the particle size distribution being controlled, i.e., while controlling the particle size distribution, characterized in that while crystals are being precipitated through crystallization by vacuum concentration, the temperature of the slurry is periodically varied up and down during the concentration of the crystallization mother liquor, i.e., the said slurry.